It has been a problem of a conventional image processing apparatus that the sharpness or an input level of an image input section composed of an input device such as CCD (Charge Coupled Device) varies depending on individual apparatuses. For this reason, when shipping image processing apparatuses, if a region judgement (judging whether a region is a character region, a spot region, and a photographic region) using a reference document is carried out with respect to individual apparatuses, the result of the judgement is not coincident with respective ratio specified in the reference document. Here, the reference document includes a character region, a spot region, and a photographic region at such a specified ratio.
As a countermeasure, in order to compensate the variations in the input devices of the image processing apparatuses, an improvement in the mechanical and dimensional accuracy of the input device and correction such as an adjustment of an input level (shading correction) are made. However, the improvement in the mechanical and dimensional accuracy involves a substantial increase in cost. Also, the shading correction has not reached a level where the variations in the input devices are completely corrected. This results from the fact that the accuracy of a reference white plate density and the accuracy of mounting the white plate are not high enough.
In order to improve an image quality of an image processed by the image processing apparatus, while still having the problem of variations in the input devices, it has been a focus of ongoing research to develop technology which allows (1) separation of the character, photographic, and spot regions of a document which is subjected to image processing and (2) an optimum process to be carried out according to the feature of each region.
For example, Japanese Unexamined Patent Publication No. 133159/1994 (Tokukaihei 6-133159) discloses an arrangement of an image processing apparatus for accurate detection of a spot region with a simple arrangement. According to this arrangement, first, a local change in image data of a region is detected with respect to a main-scanning direction and a sub-scanning direction, respectively. Secondly, it is determined whether the region is a spot region based on pixels, in the region, having detected changes above a predetermined value. Finally, a smoothing filter is applied to the spot region so as to prevent moire, and an MTF correction filter (edge enhancement filter) is adopted to a non-spot region so as to sharpen the image.
Despite the fact that the quality of region separation has been improving in the past years, there has been no improvement as to the correction of variations in input devices. This presents a problem that the image quality of an image that has been image-processed deteriorates since the accuracy of region separation greatly changes depending on the degree of variations in the input devices. That is to say, despite the fact that an optimum process is carried out for each of the separated regions, the image quality deteriorates due to low accuracy of the region separation.
Even in the arrangement disclosed in the afore-mentioned publication, the accuracy of region separation greatly changes depending on the degree of the variations in the input devices. Because this arrangement does not have means for adjusting the result of the region separation so as to compensate for change in an input device and the variation in the input devices, it presents a problem that a high image quality is achieved in one apparatus, while posing a problem of a coexisting region in another apparatus in which a spot region and a non-spot region coexist. Such a coexisting region causes that the resulting image is likely to have a poor image quality.